Traditionally, and almost certainly as a result of the fact that early
research in this area was done with animals, performance in matching tasks
has been ascribed to the development of unmediated stimulus selection.
But over the years, in studying these performances with humans, I have
encountered much evidence suggesting that many complex matching performances
are indeed mediated, and by a variety of verbal operants.

Figure 1

Before describing the role of mediation, it helps to have a clear account
of unmediated selection, and so here is an account of unmediated stimulus
selection in a matching to sample task. Figure 1 shows two cases
where stimuli determine the selection of other stimuli. In the first
case there is no consistent relation between the stimuli, and the correct
stimulus is arbitrarily assigned. In this case, the subject is reinforced
for selecting a triangle comparison in response to a horizontal line sample.
In the second case there clearly is a relation -- identity matching --
here the subject selects a triangle comparison for a triangle sample. Typically,
performance in both of these tasks would be accounted for in the same way,
as illustrated on the right side of the figure. According to this account,
the probability of a pointing response to a particular comparison is increases
in the presence of a particular sample as a result of a history of reinforcement
for such responses. The problem, however, is that this account treats both
the identity and the arbitrary matching relations as if they were both
arbitrary. This approach doesn't recognize that in one case the sample
and comparison are the same shape. The identity relation is ignored!
Clearly, what is needed here is an account of stimulus control in the matching
to sample task that recognizes formal relations between stimuli,
and thereby provides a rationale for generalized responding based on these
relations.

Figure 2

The next figure illustrates this account. This is a figure from an experiment
published several years ago in JEAB. The task was delayed identity matching
with non-verbal retarded children, and it used hand-signs in order to make
all components of the behavior overt and directly measurable. Subjects
first learned hand-signs for the four shapes of the training set -- as
illustrated on the top of the figure. The subjects then learned to use
these hand-signs to mediate an identity matching performance. This
was done as shown in the lower part of the figure. First, the subjects
learned to make the appropriate hand sign to whatever shape was the current
sample. Subjects also learned to maintain the hand-sign over the delay
interval, and into the presentation of comparisons. And finally subjects
learned to make the current hand-sign to that comparison for which the
hand sign was appropriate. When all of these components were performed
correctly, this necessarily produced an identity match.

Figure 3

After this performance was trained, generalized identity matching was tested
with the transfer set stimuli shown in the top of the slide. But generalized
identity matching did not appear with these shapes. Next, the hand-signs
shown here were trained to each of these shapes and then, in a subsequent
retest, generalized matching appeared immediately. Thus, once subjects
acquired hand-signs to these novel stimuli, the matching performance generalized.
Why this pattern of behavior produces immediate generalization can be simply
explained in terms of the verbal relations between hand-signs and stimuli
that are illustrated in this next picture.

Figure 4

This figure shows identity matching with two different stimuli: the two
dots from the training set, and the horizontal line from the transfer set.
First off, the hand-signs to the sample shape are described as tacts. Rehearsing
the hand sign through the delay interval and into the comparison presentation
can, I think, without stretching the definition too far, be labeled as
a self-echoic repetition of the sample tact, Finally, making the hand sign
to a comparison is again a tact.

Clearly, in both of these examples, the correct comparison stimulus
is unique because it is the only comparison that allows the subject to
make one hand sign that is both a self-echoic with respect to the sample
tact, and also itself a tact for that comparison. An accurate hand sign
of any other comparison would require the subject to change from the rehearsed
hand sign. As you can see in the two examples, only the comparison identical
to the sample allows one hand sign to be jointly under both tact and self-echoic
control. I call this unique form of stimulus control joint control. It
is unique because it can only occur with one comparison on each trial.
But as you can see by looking at the two examples, the event of joint control
is common to both cases where a comparison is encountered that evokes the
rehearsed hand sign. And it is this joint control event that evokes the
selection response. Probably the most important question these considerations
raise is with regard to their relevance to the description of ordinary
behavior, when it is not accompanied by overt mediating responses.

Figure 5

The next example illustrates how this might work. Let us assume a delayed
matching task in which the subject must select comparisons in response
to spoken descriptions -- that is, to verbal samples. In this slide, the
comparisons are white and black squares and circles. Thus given the verbal
sample "White circle", the subject emits it as a self-echoic while perusing
the comparisons. At the actual white circle, what is being said as a self-echoic
repetition of the sample, now also functions as a tact. That is, at this
point the subject is saying the description white circle both as an echoic
repetition of what the experimenter said, and also as a tact for the white
circle itself.

Thus the correct comparison is unique -- it alone evokes joint tact
self-echoic control. Now, having laid this out, the question naturally
arises as to the evidence for this sort of control. And so I would like
to report today on some experiments we have concluded examining the performance
illustrated in the last slide: the selection of complex, multi-dimensional
stimuli in response to spoken phrases, or perhaps less formally, the selection
of complex objects in response to their descriptions.
The goal of these experiments was to look for evidence for the operation
of each of the two legs of joint control. And so the first experiment looked
at the role of tact behavior, and the second looked at the role of the
self-echoic.

Figure 6

Both of the experiments used successive-comparison presentations. And so
as shown here, on each trial, after the spoken sample was presented, the
comparisons appeared one at a time. The subject could either select the
comparison by pressing it, or press the black square to remove this comparison
and produce another. There were six comparisons. If the subject continued
to press, the black square, the comparisons simply repeated until one was
selected.

Figure 7

The subjects were normal 4 and 5 year old children. The stimuli were constructed
with the shapes, borders and colors illustrated in the slide. The actual
color of each stimulus is given below it in parenthesis. The stimuli were
given unfamiliar names. The colors purple, aqua and brown were named King,
Pond, and Leaf respectively; the shapes were named Bus, Trap, and Flag;
the Borders were called Sol, Point, and Check. Typical stimuli generated
from this set are illustrated at the top of the slide.

The first stimulus marked with an asterisk, would be described as King-Bus-Point
because it is purple, has the internal shape named bus, and has the border
named Check. Six stimuli were constructed for a training set. The top of
the figure illustrates some of them. In Phase 1 of training, subjects were
trained in the successive comparison procedure I described. They were trained
to select each of the six stimuli when given a spoken description as a
sample. Learning here was very slow. It took over 400 trials for the children
to learn to select the six stimuli in response to their descriptions.
When they finally did learn this, the children were immediately given
the naming test listed here as Step 2. Here, they were shown one stimulus
at a time, and simply asked give its description. No child could fully
describe more than two of the stimuli, and most could only give a complete
description, for one stimulus. This, even though they had just spent 400+
trials learning to select these stimuli in response to the descriptions,
they could not give the descriptions from looking at the stimuli. No symmetry
was apparent here.

In Step 3, subjects were given a test for generalization. New stimuli
were constructed from novel combinations of the same features, as shown
in the bottom of the figure, and subjects were required to select each
novel stimulus when given its spoken description. But we found that subjects
could not do this. There was no generalization from initial training to
these novel combinations. Apparently teaching comparison selection with
the training set was, in and of itself not an antecedent adequate to produce
generalized selection. So the question we addressed next was what was missing
from the subjects repertoire that would produce generalized selection to
the novel combinations. In light of our earlier findings illustrating the
efficacy of joint control in producing generalized matching, this was the
route we chose to explore. We proceeded to bring selection behavior under
joint control.

Figure 8

In Step 4, subjects were taught tacts for each of the colors, shapes and
border patterns. So here the colors, shapes and borders were each presented
alone, and subjects learned the names for each feature. However, this training
produced no improvement in Step 5 in the retest for generalization.

Figure 9

As illustrated on this next slide, in Step 6, Subjects were trained to
name the elements contained in each of the training-set stimuli. To do
this, the subject was shown a color and asked to tact it. Next, a shape
in that color appeared, and the subject had to name the color and the shape.
Then a border appeared around the colored shape, and the subject had to
name the color, shape and border. But even after this training was complete,
and subjects could now describe all of the training set stimuli, generalized
selection to novel stimuli did not appear in the generalization test. Subjects
still responded here at chance levels.

We suspected that this was somehow due to the fact that the stimuli
and their names were familiar, and that subjects could repeat the names
as memorized intraverbals rather than by actually generating the names
by tacting each element of the training-set stimuli. And so in the next
phase, the tact-training procedure was repeated, but with some novel, unfamiliar
combinations of color, border and shape. Here, the subjects could not just
repeat the names they had heard over 400 times in training, but rather
were forced to literally construct the names for the stimuli by tacting
each element.

Apparently this was the key. Because when given the generalization test
in Step 9, the subjects immediately performed virtually without error.
It seems that the generalized selection of novel combinations of color,
shape and border could only occur after subjects learned to actually emit
appropriate tacts for each of the elements of the novel stimuli. It thus
appears that generalization depended on subjects learning to accurately
tact novel comparison stimuli.
From a larger perspective, we see here that at least one verbal relation,
the tact leg of joint control, was necessary for generalized matching.
As to the second verbal relation that can comprise joint control, the
self-echoic, that was explored in a second experiment.

Figure 10

As we see in this slide, the procedure here replicated the last experiment.
The major difference was that here the names were familiar. This slide
shows four of the training-set stimuli. The first stimulus would be Blue-Boy-Box.
The second is Purple-Clown-Circle. After learning in Step 1 to select all
the training set stimuli in response to their names, just as in the first
experiment, the subjects were given the Naming Test in Step 2, and asked
to give the names for each of the stimuli. This they could do with no errors,
unlike the prior experiment, where no subject at this point could describe
more than two of the stimuli.

Thus, subjects here could apply tacts they had learned pre experimentally
for these stimuli. And given this pre experimental history, the generalization
seen in Step 3 was dramatic, but of course, not unexpected. Thus, after
learning to select combinations with names like Blue-Boy-Box and Purple
Clown-Circle, there was immediate generalized selection to stimuli comprised
of novel combinations of these same names.

The next phase got to the heart of the matter: the role of self-echoic
rehearsal was explored. This phase took advantage of the fact that with
successive presentations, the comparison stimuli only appeared one at a
time. Thus, on some trials the correct comparison appeared immediately
after the sample description was pronounced. But on other trials 2, 3,
4, or even 5 comparisons might intervene before the subject saw the correct
comparison.

To prevent subjects from rehearsing the sample description while waiting
the correct comparison to appear, the subjects were now required to read
numbers from a constantly changing display that appeared on the screen
just above the comparisons. Thus, as before, subjects would hear the sample
spoken and then see the comparisons one at a time, and as before, they
could select a comparison by pressing on it, or see the next comparison
by pressing the black square. but now, while seeking the correct comparison,
they had to read the numbers appearing on the screen. Presumably, this
impeded the rehearsal of the sample description.
As soon as this data was collected, these same subjects were then given
matching to sample training, but with the 6 stimuli with unfamiliar names
like King-Bus Point. Once they learned to select the 6 training set stimuli
in response to their descriptions, we examined the effect of preventing
rehearsal on matching accuracy.

Figure 11

This next slide shows a summary of the effect of preventing rehearsal on
selection performance with the two types of names: familiar names like
Blue-Boy-Circle and unfamiliar names like King-Bus-Point. Notice, that
we are not looking at generalization data here, these data describe the
accuracy with which subjects selected training-set stimuli in a successive
comparison presentation task where rehearsal is prevented.

As we see, there isn't much difference with comparisons in the
first three positions, but after that, with the familiar names like blue-boy-circle,
the accuracy of a correct selection declined as a function of the number
of comparisons intervening between the sample and the correct comparison
-- This trend was found to be statistically significant. That is, subjects
made fewer correct selections when 5 or 6 comparisons intervened between
the sample and the correct comparison, than when only 1 or 2 intervened.
This strongly implies that some self-echoic rehearsal process mediated
selection here.

Interestingly, with the unfamiliar names like King-Bus-Check, the gradient
is flat! With these stimuli, subjects maintained their selection accuracy
regardless of the number of comparisons intervening between the spoken
sample and the correct comparison. Thus, they were just as accurate selecting
comparisons that appeared in the 5th or 6th position as in the first or
second. Apparently, whatever the subjects were doing with these stimuli,
it did not involve vocal rehearsal of the stimulus names. Here then we
have evidence for a direct, unmediated form of stimulus control.

TYPE OF SAMPLE

Familiar names (e.g., Blue-Boy-Circle)

Unfamiliar names(e.g., King-Bus-Point)

Acquisition of initial
matching-to-sample performance

Very rapid

Very slow

Naming Test performance

Errorless - Subjects named all training set stimuli correctly

Poor - Subjects named no more than 2 out of 6 correctly

Generalization to novel stimuli

Immediately after initial training

None - until trained to tact stimulus elements.

Effect of rehearsal prevention on selection accuracy

Accuracy deteriorated with increasing distance between samples and
comparisons

Accuracy was maintained

Initial form of stimulus control

Joint Control

Conditional discrimination

Figure 12

Let us then summarize and conclude here. Taken together, these data
suggest that, in fact, two forms of stimulus control were operating. One
is the form of stimulus control typically ascribed to SD, and the other
is based on the joint operation of two verbal relations. The properties
of each are illustrated in this table. The 3rd column describes characteristics
of behavior with unfamiliar names. The middle column describes characteristics
of behavior with familiar names. Looking at the unfamiliar names
first. With unfamiliar sample names like King-Bus-Point, initial acquisition
was very slow, and in the naming test, performance was poor. Subjects could
not tact more than 2 out of 6 stimuli. Also, generalization didn't appear
until after specific training to tact the stimuli. In addition, rehearsal
prevention produced a flat gradient. This is exactly what would be expected
if the initial form of stimulus control over the selection response involved
the spoken sample as a conditional stimulus exerting unmediated stimulus
control over comparison selection.

On the other hand, with the familiar names, shown in the middle column,
initial acquisition was very rapid, and in the naming test subjects could
name all of the stimuli, thereby showing that tacts were available. Furthermore,
generalization here was immediate, and with rehearsal prevention, there
were declining gradients of accuracy -- thereby illustrating a dependency
of selection on self-echoic rehearsal. And so, while it is
indeed possible for subjects to learn a matching to sample task as a simple,
unmediated conditional discrimination, it also appears that subjects may
learn additional verbal relations whose interaction mediates generalized
relational responding.